Gas liquefaction process



Dec. 31, 1963 c; G. FILSTEAD, JR

GAs LIQUEFACTION PRocEss Filed April 18, 1960 United States Patent O 3,116,135 GAS HQUEFACHI EPRCESS Charles G. Fiistead, Er., Westport, Conn., assigner to Couch international Methane Limited, Nassau, Bahamas, a corporation of the Bahamas Filed Apr. i3, 1960, Ser. No. 22,746 i2 Ciaims. (El. o2-lil) This invention relates to a gas liquefaction process and, more particularly, to a liquefaction process which is economical for operation from the standpoint of balance between the power requirements and cost of equipment.

It is an object of this invention to provide a new and improved gas liquefaction process which is economical in operation from the standpoint of power requirements and cost of equipment.

Another object is to produce a process for the liquefaction of natural gas wherein use is made of an expansion cycle for making a large volume of dry gas available for purposes of lrefrigeration in gas-to-gas heat exchange with a compressed portion of the natural gas, and wherein use is made in combination therewith of a cascade cycie for liquefaction of the gas by heat transfer from the large amount of cold dry gas made available by the aforementioned expansion cycle.

Another object is to provide a liquefaction process of the type described wherein a portion of the gas is expanded for making a large amount of the cold gas available for refrigeration and wherein the expansion is achieved under conditions to avoid the production of a wet gas so that most efficient utilization can be made of the expansion equipment and maximum production can be made of cold gas for refrigeration, thereby to provide a more efficient and economical cycle.

These and other objects and advantages of this invention will hereinafter appear and for purposes of illustration, but not of limitation, an embodiment is shown in the accompanying drawing, in which- FIGURE 1 is a process flow sheet illustrating the practice of this invention.

This invention has application chieily to the liquefaction of natural gas but it can be employed also for use in the liquefaction of other gases such as air, nitrogen, helium, oxygen and the like. Natural gas, as the term is used herein, is composed mostly of methane while the remainder, making up to by weight of the gas, may be composed of acid gases, nitrogen, aromatics and heavier hydrocarbons such as ethane, propane, butane and the like. These other gases may be removed from the stream of natural gas by conventional cleanup equipment prior to introduction into the liquefaction cycle, or during the liquefaction cycle.

Gas liquefaction systems can be divided generally into two main categories. One is based on an expansion cycle wherein gas at high pressure is allowed to expand, preferably with work, with a corresponding reduction in temperature until a wet gas is produced wherein the condensed portion can be removed from the dry gas residue. The latter can be recompressed and recycled with makeup gas through the expansion system. The compressed gas is usually passed in heat exchange relation with a refrigerant to remove heat of compression and to cool the gas under pressure, whereby a higher percentage of wet gas is produced during the subsequent expansion step or steps. The use of expansion cycles of the type described in the liquefaction of natural gas has been illustrated in the previously issued Patents Nos. 2,679,145 and 2,705,406.

The cascade system is based upon the use of heat exchangers arranged progressively to reduce the temper- CTL ICC

lillii Patented Dee. 3l, l:

ature of the gas to liquefaction temperature, at which point all of the gas is converted to a liquefied state. Heat exchange for refrigeration is usually carried out while the gas is at some higher pressure above atmospheric pressure, whereby the temperature conversion is at a higher and more accessible level. The compressed and liquefied gas is then let down to a lowered pressure for storage and transportation. During the reduction in pressure, some of the liquid will be flashed olf as a gas with accompanying reduction in temperature.

The selection of a cascade cycle or an expansion cycle for liquefaction depends upon a number of factors, including the composition of the gas, the power requirements, the space available for the equipment,I the cost of raw materials, the type of equipment, and the like.

This invention is addressed to a liquefaction process which is based upon the use `of a combination of a cascade cycle and an expansion cycle. More specifically, it is a concept of this invention to process the natural gas to be liquefied in a manner wherein a portion thereof, including the feed gas, is subjected to an expansion step for the purpose of making a large amount of cold gas available at least expense for refrigeration purposes, while other portions of the natural gas are :subjected to heat exchange while at high pressure for conversion of the gas -to a liquefied state, and wherein the streams are interconnected in such a way that the feed gas is u-ltimately converted to liquid product in the liquefaction cycle.

The basic concepts of this invention may be outlined as follows:

(a) Compression of the main gas stream to a superatmospheric pressure;

(b) Subdividing the compressed main stream of gas into a refrigerant portion and a liquefaction portion;

(c) Passing the refrigerant portion in heat exchange relationship with an outside refrigerant to remove heat and to cool the gas;

(d) Expanding the cooled and compressed refrigerant portion of the gas stream to a lower pressure with corresponding reduction in temperature but `to a temperature level above liquefaction temperature of the gas;

(e) Passing the expanded refrigerant portion of the main gas stream in heat exchange relation with the compressed liquefaction portion to cool the latter to liquefaction temperature, whereby the latter is converted to a liquefied state;

(f) Dropping the pressure of the liquefied gas to a lower pressure for storage with the resulting release of some of the liquefied gas as a vapor flash with corresponding reduction in temperature of the liquid;

(g) Recycling the iiashed gas for joinder with the expanded gas of the refrigeration stream to make up a part of the main gas stream; and

(h) Introduction of feed gas in an amount corresponding to the condensed portions removed from the system and joining the feed gas with the refrigerant gas -to make up another part of the main gas stream.

The foregoing describes the main concepts of this invention. It will be understood that various modifications may be made with respect to the details for introduction of the feed gas from the well into the refrigeration stream, either prior to the expansion cyole or subsequent thereto, depending upon the pressures of the two streams. it will be apparent, also, that 4the flash from the let-down liquefied portion may be removed from the system for use in refrigeration or in the generation of power, in which event the feed gas lwill make up for such differences as would be effected by the removal.

For a more detailed description of the process, reference will now be made to the process flow sheet wherein the main gas stream in line llt) is processed through multistage compressors liz to raise the main stream of gas to a high pressure. In a multiple-stage compression system of the type described, it is desirable to pass the gas through intercooilers 14 between stages to remove heat of compression to increase the ethciency of the operation and to minimize the entrainment of lubricant `as a mist or otherwise in the `gas stream. The intercoolers preferably comprise 'water coolers wherein available water is passed in heat exchange relationship with the main gas stream between stages of the compression cycle.

The compressed main gas stream in line l5 is subdivided into one increment, which will hereinafter be referred to as the liquefaction stream, and another increment, twhich will hereinafter be referred to as the refrigeration stream. The liquefaction stream is caused to How through line 13 to a heat exchanger Ztl wherein the liqueyfaction stream of gas is passed in heat exchange relation with the cold gas of 4the refrigeration stream to reduce the temperature of the liquefaction stream to lique-faction temperature at the pressure level of the gas whereby substantially :all of the gas in the liquefac-tion stream is condensed to the liqueiied state.

The liqueed gas, at high pressure, leaves the heat cxchanger 29 through line 22 and it may be either collected as a liquid in the receiver 2lior bypassed to the expansion valve 26 where the pressure is let down to a lower pressure at which the liquid product is to be stored Ifor use or transportation.

In the preferred practice of this invention, the high pressure liquid is let down to atmospheric pressure or slightly above. This let down in pressure is accompanied by conversion of some of the liquid t0 a gas with corresponding reduction in temperature to the boiling point temperature of the liquid at the lower pressure conditions existing. For methane, this would be about 258 F. at atmospheric pressure. The let-down stream is advanced through line 218 to :a receiver 3@ wherein the iashed vapors are separated from the liquid product which can be drained through line 32 for transfer to a storage vessel or for transportation to a conveyance means.

The refrigeration stream is caused to flow through line 34 to one or more heat exchangers, identied by the numerall 36, for passage in heat exchange relation with refrigerants to extract heat from the gas at a level Where heat transfer is most eiicient and where horsepower of refrigeration per to-n of gas is lower, thereby to make use of refrigeration where greatest efciencies can be practiced for heat extraction. The cold compressed gas from the heat exchangers 36 is advanced through line 3S to an expander 40;.

It the feed gas from the well is at a pressure corre spending to `that or the refrigeration stream, it can be introduced into the system through line 42 for joinder with the refrigeration stream in line t4 in advance of the expander to blend the feed gas Iwith the cold and compressed refrigeration stream. It is preferred, however, to introduce the feed gas through line 42a for joinder `with the main stream intermediate a stage of compression, such as before the last compression.

If the feed gas is derived from a well having suliicient casinghead pressure, it will be unnecessary to compress the gas to the desired level for blending with the main stream. If lthe pressure from the casinghead has been insuiilcient or if the pressure has been used during the interventing cleanup to remove acid gases, heavier hydrocarbons and the like, the feed stream may be introduced into the system by connecting line 42a to lines feeding .finto earlier stages of compression, depending on the casinghead pressure. It is preferred to clean up the feed gas prior :to introduction into the liquefaction cycle, but removal of all of the acid gases, water or heavier hydrocarbons in advance of processing through the liquefaction system is unnecessary. For purposes of this description, it will be `assumed that the feed stream has been cleaned i up and that it is at a pressure and temperature corresponding to the cooled and compressed main gas stream in the line feeding into the last compression stage.

The combined feed land refrigeration stream in line ifi is expanded ywith work through an expander turbine 4t) to a lower pressure, with an accompanying reduction in temperature. The amount of expansion and/ or the temperature of the stream prior to expansion is calculated to provide an expanded stream of gas which remains above the condensation temperature for methane, whereby a large volume of cold vgas is Irnade .available for purposes of refrigeration. Controlled expansion to a level where condensation is avoided further increases the amount of cold refrigeration gas, yand the avoidance of condensate during the expansion cycle eliminates the erosions of the expander parts which might otherwise take place.

In the illustrated process flow sheet, the cold flash from the receiver 3d is returned through line 4.6 for joinder with the cold expanded gas in line 45S for passage together through line 5t to the heat exchanger 2d. Thus, the cold gas passed in heat exchange relationship for condensa-tion of the liquefaction stream of gas constitutes the main gas strear. made up of the feed gas `from the well, Ithe flash from the expanded condensate, and the refrigeration stream which has been divided out `from the main gas stream subsequent to recompression. The gastto-gas heat exchange made available by the expansion of the refrigeration stream toy a cold dry gas provides for most etlicient heat transfer to eect condensation of the liquefaction stream.

`lit will be apparent that the amount of feed gas from the well will correspond to the amount of gas that is removed from the system as liquefied product, and that the feed gas from the 'well first forms a part of the refrigeration stream before becoming a part of the liquefaction stream whereby a continuous cycle is established.

An important iconcept of this invention resides in the optimum efficiencies secured wherein less horsepower, in the form of horsepower of refrigeration, is required to convert a unit volume of gas into a liquid by comparison with conventional expander cycles and less equipment is required by comparison with conventional cascade cyvcies. This improved eciency is derived in part from a number of sources, including the use of a high level of refrigeration at the point where the refrigeration stream is reduced to a lower temperature subsequent to recompression but before expansion to enable removal of the greatest amount of heat with the least amount of power. The improved efciency is also traceable, in part, to making a maximum amount of cold gas available for refrigeration when passed in gas-to-gas heat exchange with the liquefaction stream by utilizing a portion of the compressed stream of gas for refrigeration purposes and by expansion of this stream under conditions wherein a maximum amount of cold gas is produced, as distinguished from a lesser amount which would otherwise be secured if liquefaction took place. Expansion under controlled conditions to avoid the production of a `wet gas also provides for more edicient expansion and less erosion on the blades of the expander.

Thus, there is provided a liquefaction process embodyfing new and novel concepts which makes use of a combination of cascade and expansion systems and which leads to greater eiciencies in operation and utilization of less power and equipment for purposes of liq-uefaction.

A separator may be introduced in line Sti prior to the heat exchanger 2@ for the removal of any heavier hydrocarbons and other condensate which might form in the expanded main stream of gas. Similarly, some of the compressed gas from either of the streams in lines `18 or 34 may be bled off for use in the generation of power to supplement the power made available from the expander iti and which can be used to operate the compressors and the like.

By way olf example, starting with a main stream of gas at 70 F. and 14.7 p.s.i.a., flowing at a rate of 1641 pounds per minute through line l0, the gas is processed through the 6 stages of compression with water cooling between alternate stages to remove heat of compression. The feed gas at 697 p.s.i.a. and flowing at a rate of 878 pounds per minute is joined with line 121 and undergoes the last stage compression. The stream of gas issuing from the -last compression stage and intercooler will have a pressure of about 1515 p.s.i.a. at 90 F. This stream is divided to flow about 1491 pounds of gas per .minute through line 34 to constitute the refrigeration stream, while the remainder corresponding to 884 pounds per minute ris cir* culated through line 1S as the liquefaction stream. The fuel gas at a rate of 14-4 pounds per minute and a pressure of 32.4 p.s.i.a. is taken from the discharge of the fourth stage compressor, and through line 41.

The liquefaction stream in line 13 issues as a condensed natural gas from the heat exchanger at a pressure of 1515 p.s.i.a. and at a temperature of -Z15 F. When -let down to atmospheric pressure (14.7 p.s.i.a.), the liquid collected in the receiver will have a temperature of about 259 F. and will be introduced at a rate `of about 734 pounds per minute.

The dry gas at the same pressure and temperature will llow at a rate of about 150 pounds per minute through line 46 for joinder with the cold gas issuing at about atrnospheric pressure from the expander 40.

The refrigeration stream in line 34 is reduced to a temperature of about `120 F. upon refrigeration through the heat exchanger 36 wherein the gas can be passed in heat exchange relationship with refrigerants operated, for example, on an ethane or propane cycle, or both. The refrigeration stream at 1500 p.s.i.a. and at +c F. is expanded in the turbine 4)` to atmospheric pressure (14.7 p.s.i.a.), with a corresponding reduction in temperature to about -258 F. The joinder of the flash with the expanded stream of gas results in a main gas stream having a temperature of about -258 F. and flo-wing at atmospheric pressure at a rate of about 1641 pounds per minute.

Thus the cycle is completed with the main gas stream being processed through the multi-stage compressors after having been passed in gas-to-gas heat exchange with the liquefaction stream in the heat exchanger 20.

.lt will be understood that the above temperature and pressure conditions may be varied over a fairly wide range depending upon the pressure of gas available or the type of gas or the amounts of gas to be liquefied. It will be lfurther understood that more than one refrigeration step may be employed for reducing the temperature of the retrigeration gas in unit but it would be undesirable to reduce the gas to a temperature at which condensation would take place upon expansion of the gas to the desired lower pressure for the production of a large volume of gas to be passed in heat exchange relationship with the liquefaction portion of the rnain gas stream. Such additional refrigeration units may be arranged in series or in parallel for the treatment of the same or separate increments of the gas respectively.

It will be understood that various changes may be made in the details of construction and operation without departing from the spirit of the invention, especially as defined in the following claims.

l claim:

1. In the process for the liquefaction of natural gas composed mostly of methane, the steps rof supplying the gas continuously as a main process stream at high pressure and at relatively high temperature, continuously rernoving a portion of the high pressure gas `from the main process stream for use as a refrigeration gas, refrigerating the portion of the gas removed from the ymain process stream to reduce the temperature thereof Without noticeable reduction in pressure bypassing it in heat exchange relation with an external refrigerant, expanding the removed and refrigerated portion of the gas to a lower pressure with corresponding reduction in temperature to a temperature below that at which the remaining gas in the main process stream is liqueed at the pressure of the main process stream but to a level above condensation temperature for the `methane component of the expanded refrigeration portion of the gas thereby to produce a large volume of cold gas for refrigeration, passing the cold expanded portion of the gas in heat exchange relation with the remainder `ot the process stream at high pressure to reduce the temperature thereof to `a level to condense the remainder `of the `main process stream at the pressure conditions existing, dropping the pressure of the condensed portion of the lmain process stream whereby some of the condensed gas is Jdashed oli with a further reduction in temperature, separating the condensate from the liashed gases, recompressing the expand-.ed refrigeration portion of the stream `and introducing it into said main process stream to the pressure of the main process stream continuously to form a part thereof.

2. The process as claimed in claim 1 which includes the step of combining the ashed gases separated from the condensate with the expanded refrigeration portion removed from the main process stream prior to passage thereof in heat exchange relation with the remainder of the rnain process stream thereby to include the flashed gases as a part of the main process stream upon recompression.

3. In the process for the liquefaction of natural gas composed mostly of methane, the steps of supplying the gas continuously as a main process stream :at high pressure and at relatively high temperature, continuously removing a portion of the high pressure gas from the rnain process stream for use in refrigeration, passing the portion of the gas removed from the main process stream in heat exchange relationship with an external refrigerant to reduce the temperature of the gas without noticeable reduc-tion in pressure, expanding the removed and refrigerated portion of the gas to a lower pressure with corresponding reduction in temperature to a temperature below that at which the remaining gas in the main process stream is liquefied at the pressure of the main process stream but to a level above condensation temperature for the methane component of the expanded refrigeration portion of the gas thereby to produce a large volume of cold gas for refrigeration, passing the cold expanded portion of the gas in heat exchange relationship with the remainder of the process stream `at high pressure to reduce the temperature thereof to a level wherein the gas in the remaining portion of the main process stream is condensed under the pressure conditions existing, reducing the pressure of the condensed portion of the main process stream whereby some of the condensed gas is flashed olf with -a further reduction in temperature, separating the zcondensate at lower pressure from the flashed gases as product, introducing an amount of feed gas into the refrigeration portion separated from the main process stream in an amount corresponding to the amount of gas removed from the main process stream as condensate and reconipressing the expanding refrigeration portion together with said feed gas to the pressure of the main process stream and introducing these compressed gases into the main process stream to form a part thereof.

4. The process as claimed in claim 3 which includes the additional step of combining the flashed gases separated `from the condensate with the refrigeration portion of the gas separated from the main process stream prior to passage of the refrigerated portion in heat exchange relationship with the remainder ot the main process stream.

5. in the process for the liquefaction of natural gas composed Amostly of methane, the steps of supplying the gas continuously as a main process stream at high pressure, continuously removing a porti-on of the high pressure gas from the main process stream for use in refrigeration, passing the portion of the gas removed from the main process stream in heat exchange relationship aliarse with an external refrigerant to reduce the temperature thereof without noticeable reduction in pressure, expanding the removed and refrigerated portion of the gas with work to a lower pressure with a corresponding reduction in temperature to produce `a large amount of cold gas for refrigeration but `without reduction of temperature to a level for condensation of the methane portion thereof, passing the cold expanded portion of the gas in heat exchange relationship with the remainder of the process stream at high pressure to lcondense the main process stream at the pressure conditions existing, letting down the pressure of the condensed portion of the main process stream whereby some of the `condensed gas is flashed olf with a corresponding reduction in temperature, separating the condensate from the flashed gases, joining the cold flashed gases with the expanded portion of the main process stream prior to passage thereof in heat exchange relationship with the remainder of the main process stream for condensation, recompressing in a plurality of stages the expanded portion of the main process stream and the flashed gases, introducing these cornpressed gases into the main process stream to form a part thereof and introducing an amount of feed gas corresponding to the amount of gas removed from the main process stream as condensate interstage of the compression cycle to make up the main process stream whereby the feed gas is subject to compression in the linal stages of recompression of the main process stream and whereby a main process stream at high pressure is continuously provided.

6. in the process for the liquefaction of natural gas composed mostly of methane, the steps of supplying the gas continuously as a main-process stream at high pressure, continuously removing'a portion of the high pressure gas from the main process stream for use in refrigeration, passing the portion of the gas removed from the main process stream in heat exchange relationship with an external refrigerant to reduce the temperature thereof Without noticeable reduction in pressure, expanding the removed and refrigerated portion lof the gas with work to a lower pressure with a corresponding reduction in temperature to produce a large amount of cold gas for refrigeration but -without reduction of temperature to a level for condensation of the methane portion thereof, passing the cold expanded portion of the gas in heat exchange relationship with the remainder of the process stream at high pressure to condense the main process stream at the pressure conditions existing, letting down the pressure of the condensed portion of the main process stream whereby some of the condensed `gas is flashed off With a corresponding reduction in temperature, separating the condensate from the `trashed gases, joining the cold `iiashed gases with the expanded portion of the main process stream prior to passage thereof in heat exchange relationship `with the remainder of the main proc-ess stream yfor condensation, introducing an amount of feed gas into the refrigeration portion separated from the main process stream in an amount corresponding to the amount of condensate removed from the main process stream whereby the main process stream is continuously formed of the ilashed gas, the refrigeration gas and the feed gas, and recompressing such gases forming the main process stream to the pressure .level of the main process stream subsequent to passage of said gases in heat exchange relationship with the remainder ofthe rmain process stream at high pressure.

7. The process as claimed in claim 5 which includes the steps of intercooling the main process stream between stages of compression.

8. The process as claimed in claim 5 in which the condensed portion of the main process stream is expanded to about atmospheric pressure.

9. rthe process as claimed in claim 5 in which the refrigeration portion of the gas separated from the main process stream and the feed gas is expanded with work to about atmospheric pressure.

l0. in the process yfor the liquefaction of a gas, the steps of suppiying the gas continuously as a main process stream at high pressure, continuously removing a portion of the high pressure gas from the main process stream to provide a refrigeration portion and a condensible portion, passing the refrigeration portion in heat exchange relationship with an external refrigerant to reduce the temperature thereof without noticeable reduction in pressure, expanding the refrigeration portion of the gas to .a lower pressure with corresponding reduction in temperature but to a level above condensation temperature for the gas, thereby to produce a large volume of cold gas for refrigeration, passing the expanded refrigeration portion `of the gas in heat exchange relationship with the condensihle portion of the main process stream at high pressure to condense the portion at the pressure conditions existing, reducing the pressure of the condensed portieri whereby some ot' the condensed gas is flashed oif with a corresponding reduction in temperature, separating the Sash from the condensate, combining the ash ywith the refrigeration portion prior to passage thereof in heat exchange relationship with the condensihle portion of the main process stream, compressing a feed gas to the pressure of the main pro-cess stream and introducing the feed gas into the refrigeration portion of the main process stream prior to expansion thereof and in an amount corresponding to the amount of gas removed as condensate yfrom the condensihle portion of the main process stream, and recompressing the gases passed in heat exchange relationship with the condensible portion or" the main process stream to the pressure o1 the main process stream whereby such gases constitute the main process stream.

l1. The process as claimed l2. The process as `claimed in claim 3, in ywhich the feed gas is introduced into the refrigeration portion before the said portion is expanded,

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Ruheman, M.: The Separation of Gases, London, Oxyford University Press, 1949, Second Edition, chapter V, pages 131-135.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No; 3, 116, 135 December 31, 1963 Column 6, lines 16 and 17, strike out it `into said main process stream" and insert and introducing "stream" in line 18, same column 6 the same after Signed and sealed this 28th day of July 1964u (SEAL) Attest:

ESTON G. JOHNSON EDWARD J. BRENNER Attesting Officer Commissioner of Patents 

1. IN THE PROCESS FOR THE LIQUEFACTION OF NATURAL GAS COMPOSED MOSTLY OF METHANE, THE STEPS OF SUPPLYING THE GAS CONTINUOUSLY AS A MAIN PROCESS STREAM AT HIGH PRESSURE AND AT RELATIVELY HIGH TEMPERATURE, CONTINUOUSLY REMOVING A PORTION OF THE HIGH PRESSURE GAS FROM THE MAIN PROCESS STREAM FOR USE AS A REFRIGERATION GAS, REFRIGERATING THE PORTION OF THE GAS REMOVED FROM THE MAIN PROCESS STREAM TO REDUCE THE TEMPERATURE THEREOF WITHOUT NOTICEABLE REDUCTION IN PRESSURE BYPASSING IT IN HEAT EXCHANGE RELATION WITH AN EXTERNAL REFRIGERANT, EXPANDING THE REMOVED AND REFRIGERATED PORTION OF THE GAS TO A LOWER PRESSURE WITH CORRESPONDING REDUCTION IN TEMPERATURE TO A TEMPERATURE BELOW THAT AT WHICH THE REMAINING GAS IN THE MAIN PROCESS STREAM IS LIQUEFIED AT THE PRESSURE OF THE MAIN PROCESS STEAM BUT TO A LEVEL ABOVE CONDENSATION TEMPERATURE FOR THE METHANE COMPONENT OF THE EXPANDED REFRIGERATION PORTION OF THE GAS THEREBY TO PRODUCE A LARGE VOLUME OF COLD GAS FOR REFRIGERATION, PASSING THE COLD EXPANDED PORTION OF THE GAS IN HEAT EXCHANGE RELATION WITH THE REMAINDER OF THE PROCESS STREAM AT HIGH PRESSURE TO REDUCE THE TEMPERATURE THEREOF TO A LEVEL TO CONDENSE THE REMAINDER OF THE MAIN PROCESS STREAM AT THE PRESSURE CONDITIONS EXISTING, DROPPING THE PRESSURE OF THE CONDENSED PORTION OF THE MAIN PROCESS STREAM WHEREBY SOME OF THE CONDENSED GAS IS FLASHED OFF WITH A FURTHER REDUCTION IN TEMPERATURE, SEPARATING THE CONDENSATE FROM THE FLASHED GASES, RECOMPRESSING THE EXPANDED REFRIGERATION PORTION OF THE STREAM AND INTRODUCING IT INTO SAID MAIN PROCESS STREAM TO THE PRESSURE OF THE MAIN PROCESS STREAM CONTINUOUSLY TO FORM A PART THEREOF. 